3. Bacteriology lab

Question 1

what are common diagnostic techniques in a bacteriology lab?

Answer 1

• culture of sterile sites (blood/CSF) and non-sterile sites (urinary tract/bowel/skin)
• serology looking for an amounted immune response to infection
• molecular techniques (PCR)
• antimicrobial susceptibility testing

Question 2

give examples of first-line defence against bacteria that the body has (innate immunity)

Answer 2

• lysozyme in tears kill gram + bacteria
• mucus and cilia capture organisms and remove them
• stomach acid kills ingested pathogens
• skin is a physical barrier
• competition and toxic products from intestinal flora
• urination flushes organisms
• low vaginal pH prevents colonisation

Question 3

how is a blood culture done?

Answer 3

1. blood is taken from the patient in the most sterile way possible - skin is cleaned beforehand
2. blood goes in 2 different bottles (aerobic and anaerobic)
3. blood is incubated at body temperature and bacteria multiply
4. when bacteria reproduce they produce CO2 which causes a pH and colour change on discs at the bottom of the bottles
5. the change flags on the machine

Question 4

what happens if a blood culture is positive?

Answer 4

1. blood is removed and put onto different agar plates: blood agar, chocolate agar (haemolysed blood), MacConkey agar, neomycin agar
2. growth of bacteria is observed - most bacteria grow on blood/chocolate agar, gram - bacteria grow well in MacConkey causing a colour change
3. agar plates are incubated for 24hrs

Question 5

what is the difference between gram + and gram - bacteria?

Answer 5

gram + = thick peptidoglycan wall (stains purple)

gram - = thin peptidoglycan wall sandwiched between 2 membranes (stains pink/red)

Question 6

other than cell wall how can bacteria be classified?

Answer 6

shape - cocci, bacilli etc.

Question 7

what type of staphylococcus is found on human skin?

Answer 7

coagulase negative

Question 8

what is the coagulase test?

Answer 8

a test for the enzyme coagulase - if the test is positive the type is staphylococcus aureus

coagulase changes fibrinogen to fibrin in our blood

Question 9

what can staphylococcus aureus result in?

Answer 9

severe infections - skin/soft tissue, endocarditis, osteomyelitis

Question 10

what is the difference between group A streptococcus and streptococcal pneumonia?

Answer 10

group A are beta-haemolytic - they use up all the blood on blood agar

streptococcal pneumonia are a-haemolytic - they produce a green tinge to the blood agar

Question 11

if there is a gram - rod in the blood culture why should we be worried?

Answer 11

gram - bacteria have an outer membrane that can produce toxins and may cause septic shock

Question 12

what causes diarrhoea?

Answer 12

• bacteria: salmonella, shigella, c difficile, cholera, e coli
• parasites: amoeba
• viruses

Question 13

what is sensitivity testing?

Answer 13

looking for the point at which bacteria is resistant or sensitive to a specific concentration of drug

Question 14

what is the minimum inhibitory concentration (MIC) and how is it found?

Answer 14

MIC = the point at which bacteria start growing

there are multiple test tubes including: a control with no antibiotic, bacteria but no antibiotic, antibiotic but no bacteria. a doubling dilution is done, decreasing antibiotic at each stage

Question 15

what are beta-lactamases?

Answer 15

enzymes that bacteria have which allows them to survive against their environmental competitors

Question 16

what is important to consider when a patient arrives with likely bacterial infection?

Answer 16

• travel history
• contacting infectious disease or microbiology for advice
• sending samples for culture prior to starting antibiotics